A system for maintaining atmospheric conditions in an enclosed building within desired parameters and rapidly evacuating abnormally high amounts of airborne pollutants from localized areas of the building. Conditioned air, including sufficient outside air, is continuously admitted to the building interior from a plurality of air handling units disposed about the building to maintain the interior of the building under a slight positive pressure throughout. The air handling units, at least in areas where concentrations of airborne contaminants are generated include, in addition to outside air and building air inlets and a building air outlet, an outlet exhausting to the outside. Sensors and/or scanning units located throughout the building monitor various atmospheric conditions and transmit signals indicative of the noted conditions to a central computer. The computer, in turn, compares the observed conditions to predetermined desired conditions and controls the air handling units as necessary to maintain the conditions within desired parameters. When an abnormal airborne pollutant condition is noted in a particular area, the outside air inlet and building air outlets are closed and the building return air inlet and exhaust outlet are opened so that contaminated air is withdrawn from the region and exhausted to the outside.